The technology disclosed herein relates to a method, base station, interface for handover in a wireless communication network which include at least a first base station (eNodeB) and at least one user equipment where handover initiates when the signal quality falls below a predetermined value and where random access parameters serves as an uplink procedure to enable the UE to make handover to a suitable second base station (eNodeB).
When a mobile phone is moving, it will be traveling through different cells. If the mobile phone is not engaged in a call, it will tell the network every now and then that it has moved to another cell. If the mobile phone is engaged in a call, the call of course needs to be maintained while the phone is moving. The process of replacing communication with one cellular radio station with another is called handover. When the cellular network sees the mobile phone moving closer and closer to another cell, it will initiate the handover process, during which the call will be transferred from one cellular radio station to another.
In WCDMA the handover access is done via dedicated channel. The dedicated channel uses closed loop fast power control. At the time of handover the RNC signals the initial UL DPCCH power setting parameter to the UE. This value is used by the UE to derive its initial uplink power for radio link that is to be added at the target cell. Since power control is in operation therefore uplink physical layer synchronization of the new radio link can be achieved quickly.
In the downlink of the mobile communication system LTE or E-UTRAN the modulation format is OFDM (Orthogonal Frequency Division Multiplex) for the signal bearer and the access scheme is OFDMA (Orthogonal Frequency Division Multiple Access). In the uplink of LTE, a slightly different multiple access scheme called single carrier frequency division multiple access (SC-FDMA) is used.
The actual implementation of the technology is different between the downlink (i.e. from base station to mobile) and the uplink (i.e. mobile to the base station) as a result of the different requirements between the two directions and the equipment at either end.
One fundamental difference between UTRAN and E-UTRAN in terms of handover procedure is that in the latter system the UE during the handover access the target cell via normal RACH channel (3). In UTRAN the UE is assigned a dedicated channel during handover. This implies that in E-UTRAN it is important that the UE transmits RACH in the neighbour cell with the correct transmit power level and at the correct frame or sub-frame boundary. In order to allow UE to correctly estimate the RACH transmit power the source cell should provide power control and RACH format related parameters, which are applicable to the target cell. Furthermore these parameters should be conveyed to the UE prior to the initiation of handover.
There are two main issues in the existing system as discussed below:
In WCDMA the handover access is done via dedicated channel. The dedicated channel uses closed loop fast power control. At the time of handover the RNC signals the initial UL DPCCH power setting parameter to the UE. This value is used by the UE to derive its initial uplink power for radio link that is to be added at the target cell. Since power control is in operation therefore uplink physical layer synchronization of the new radio link can be achieved quickly.
In E-UTRAN Orthogonal Frequency Division Multiple Access (OFDMA) technology is used in the downlink. OFDM is a modulation scheme in which the data to be transmitted is split into several sub-streams, where each sub-stream is modulated on a separate sub-carrier. Hence in OFDMA based systems, the available bandwidth is sub-divided into several resource blocks or units where a resource block is defined in both time and frequency. According to the current assumptions, a resource block size is 180 KHz and 0.5 ms in frequency and time domains, respectively. The overall uplink and downlink transmission bandwidth can be as large as 20 MHz.
For the LTE uplink, a different concept is used for the access technique. Although still using a form of OFDMA technology, the implementation is called Single Carrier Frequency Division Multiple Access (SC-FDMA). In essence a mobile is allocated a sub-carrier or sub-carriers for its link to the base station and it uses these to establish the uplink.
In E-UTRAN the handover access will take place via RACH, which cannot use and will not use fast power control. This means RACH transmission power during handover access should be as precise as possible. In general the handover access on RACH should use the correct setting, parameters and power levels. This requires some exchange of information related to RACH between eNode B. In the current WCDMA system no such information is exchanged. If such information is not exchanged between eNodeBs in LTE the uplink synchronization will be delayed. Secondly the collision on RACH may also increase. Thirdly the interference will increase due to increase in RACH load. Furthermore, if the serving and target cells use different RACH format then handover access to the target cell may completely fail if the UE accesses the target cell with the RACH format used in the serving cell. An alternative and straight forward solution is that prior to handover access the UE reads system information of the neighbour cells to acquire all RACH related information of the neighbour cell. The obvious disadvantage is that it will increase the handover delay and interruption time. Another problem is that it will increase complexity in the UE due to reading of neighbour cell information in RRC_CONNECTED mode.